As is explained in U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, such products are prepared through the heat treatment of precursor articles. Thus, the three basic steps underlying the production of glass-ceramics comprise: (1) a glass forming batch, customarily containing a nucleating agent, is melted; (2) the melt is simultaneously cooled to a temperature below the transformation range thereof and a glass article of a desired configuration shaped therefrom; and (3) the glass article is exposed to a predetermined heat treatment schedule to cause the glass to crystallize in situ. Most frequently, this last step is broken down into two parts. First, the parent glass article is initially heated to a temperature in or slightly above the transformation range to develop nuclei in the glass. Thereafter, the glass article is heated to a higher temperature, often above the softening point thereof, to cause the growth of crystals upon the previously-developed nuclei.
Because the mechanism of crystallization involves the essentially simultaneous growth of crystals upon a myriad of previously-developed nuclei, the microstructure of a glass-ceramic product consists of relatively uniformly-sized, fine-grained crystals homogeneously dispersed within a glassy matrix, the crystals comprising the predominant portion of the article. Glass-ceramic articles have been generally defined as being at least 50% crystalline and, in many instances, approach 100% crystallinity. This feature of high crystallinity results in the physical properties exhibited by glass-ceramics being normally materially different from those of the precursor glass and more nearly akin to those demonstrated by the crystals.
The crystal phases developed in a glass-ceramic product are dependent upon the composition of the parent glass and the heat treatment applied to the glass. The term "nepheline" has been utilized in the literature to designate a natural mineral having a crystal structure categorized in the hexagonal crystal system and identified by the general chemical formula (Na,K)AlSiO.sub.4. It has been observed in Donnay et al., however, that the mineral nepheline exists in a wide range of solid solutions, the extent of which is not fully elucidated by the above formula (Paper No. 1309 of the Geophysical Laboratory entitled "Nepheline Solid Solutions").
A similar situation exists in the glass-ceramic art where the range of solid solution is even more extensive because the growth of crystals takes place under nonequilibrium conditions. Hence, metastable crystal glasses can be grown. Thus, the term "nepheline" is employed in the glass-ceramic art to indicate a wide range of solid solution crystal phases having general characteristics corresponding to those of the mineral. Accordingly, whereas the crystals may vary substantially in composition, they have a common diffraction peak pattern when studied via X-ray diffraction analysis. In sum, whereas any nepheline crystal will display a characteristic pattern of diffraction peaks, it will be appreciated that the spacing and intensity of the peaks may vary depending upon the nature of the crystal phase.
Research has shown, however, that if potassium ions are substituted for sodium ions in the nepheline crystal, there is a tendency for the nepheline crystal phase to convert, in part at least, to a different type of crystal known as kalsilite. This potassium-containing crystal is also classified in the hexagonal crystal system and, hence, likened to the nepheline system but having a somewhat dissimilar crystal structure, as evidenced by a different pattern of diffraction peaks in an X-ray diffraction pattern analysis. This phenomenon is discussed in "The Nepheline-Kalsilite System: (I.) X-ray Data for the Crystalline Phases", J. V. Smith and O. F. Tuttle, American Journal of Science, 255, pp. 282-305, April 1957.
U.S. Pat. No. 3,573,072 described a method for chemically strengthening glass-ceramic articles wherein the predominant crystal phase consists essentially of nepheline solid solutions corresponding generally in chemical composition to the formula Na.sub.8-x K.sub.x Al.sub.8 Si.sub.8 O.sub.32. In that formula x may vary from about 0.25-4.73. The inventive method comprised exposing such glass-ceramic bodies to an external source of K.sup.+, Rb.sup.+, and/or Cs.sup.+ ions at temperatures between about 400.degree.-950.degree. C. for a sufficient length of time to effect replacement of Na.sup.+ ions in the nepheline crystals at the surface of bodies with K.sup.+, Rb.sup.+, and/or Cs.sup.+ ions, and thereby convert said nepheline crystals into kalsilite and/or crystals resembling synthetic kaliophylite. That crowding in of the larger K.sup.+, Rb.sup.+, and/or Cs.sup.+ ions into the structure of the nepheline crystals sets up compressive stresses in an integral surface layer on the bodies, thereby imparting much improved mechanical strengths thereto. Customarily, the external source of K.sup. +, Rb.sup.+, and/or Cs.sup.+ ions consisted of a bath of a molten salt containing K.sup.+, Rb.sup.+, and/or Cs.sup.+ ions.
As is explained in that patent, the inventive article consisted of a glass-ceramic having an original nepheline solid solution crystal phase containing K.sup.+ and Na.sup.+ ions, the ratio of the K.sup.+ -to-Na.sup.+ ions being greater than 0.25:1.75 on an ionic basis, the article being distinguished in having an integral surface layer in which at least a portion of the Na.sup.+ ions in the original nepheline phase is replaced by the larger K.sup.+, Rb.sup.+, and/or Cs.sup.+ ions to develop a surface layer of modified chemical composition having a degree of compressive stress generated therein by the ion replacement. The exchange of Na.sup.+ ions with K.sup.+, Rb.sup.+, and/or Cs.sup.+ ions takes place on a one-for-one basis such that the concentration of the larger cations is greater in the surface layer than in the interior portion of the article and the concentration of the Na.sup.+ ions is greater in the interior portion than in the surface layer, those differences in concentration causing the nepheline crystal structure to expand and transform to a crystal phase with a larger unit cell volume, viz., kalsilite, thereby setting up compressive stresses.
The presence of nepheline and kalsilite solid solutions was detected by means of X-ray diffraction analyses, permitting at least a qualitative estimate of the relative proportion of each in a particular glass-ceramic article. It was also observed that the various ions, and especially the alkali metal ions, tended to appear in the crystal phase in essentially the same proportion as present in the parent glass composition. Finally, where a greater proportion of K.sup.+ ions is present than that indicated in the above formula, a different type of crystal, viz., kaliophylite, tends to develop as the original crystal phase in the glass-ceramic.
The patent emphasized that the ratio of K.sup.+ ions-to-Na.sup.+ ions on an ionic basis in the parent glass must be at least 0.25:1.75 and, preferably, greater than 1:4, in accordance with the above-cited ionic formulation. In the latter instance, an increase in mechanical strength, utilizing a K.sup.+ -for-Na.sup.+ ion exchange, can be achieved at temperatures between about 400.degree.-600.degree. C. within 24 hours. Higher temperatures may be employed, however, to increase the rate of exchange, if desired. Modulus of rupture values of 200,000 psi were observed when an exchange temperature of 730.degree. C. was utilized. The exchange of Rb.sup.+ and/or Cs.sup.+ ions for Na.sup.+ ions required higher temperatures, i.e., 750.degree.-950.degree. C., to attain the desired improved mechanical strengths. Glass compositions operable in the patented invention consisted essentially, expressed in weight percent on the oxide basis, of about 1-15% K.sub.2 O, 5-20% Na.sub.2 O, 25-50% Al.sub.2 O.sub.3, 25-50% SiO.sub.2, and 5-15% TiO.sub.2.